1. Field of the Invention
This invention relates to stable, long life sodium/sulfur electrochemical cells or batteries of the type having at least one anode chamber with anodic reactant and at least one cathode chamber with cathodic reactant, the anodic reactant and cathodic reactant being separated from each other by a sodium ion conducting solid electrolyte. More particularly, this invention relates to providing a stable, long lived cathodic current collector having a boronized surface in contact with the cathodic reactant and an additional boron source in physical proximity to the boronized current collector surface. In one embodiment, the cathodic current collector is the can, or electrode container, and therefore this invention provides stable, long life cell casings, particularly a casing which is protected from corrosion effects at the anodic potentials during charging.
2. Description of the Prior Art
Sodium/sulfur electrochemical cells or batteries of the type having at least one anode chamber with anodic reactant and at least one cathode chamber with cathodic reactant and separated from each other by a sodium ion conducting solid electrolyte and having a current collector in contact with the cathodic reactant are known to the art. Instability of such electrochemical cells or batteries due to corrosion of the cathode current collector, particularly when it also serves as the cell casing, has been recognized in the art. Such corrosion is deleterious both from the standpoint of physical change of the cell container and reaction of the sulfur in the formation of corrosion products thereby reducing the cell capacity. Various attempts to solve these problems have included coatings, liners and electrical screening. U.S. Pat. No. 4,129,690 recognizes that under the electrochemical conditions prevailing in the sulfur electrode area, even stainless steel is subject to corrosion and proposes to overcome this problem by providing a current collector of electrically conductive impermeable material chemically and electrochemically inert to the cathodic reactant, such as carbon or graphite. The structure may either be made of or coated with carbon or graphite. Another proposal for achievement of stability of the sulfur electrode container is suggested by U.S. Pat. No. 4,131,226 wherein a rigid outer container of material such as mild steel is provided with an inner liner or clad of a more corrosion resistant material such as stainless steel, molybdenum or nickel-chromium. Another proposal to reduce corrosion of the cathodic cell wall in a sodium/sulfur cell or battery is provision of a screening electrode to electrically screen the cell wall as suggested by U.S. Pat. No. 4,029,857. Still another proposal for reduction of corrosion of the cell casing is provision of a hermetic mechanical seal of glass and ceramic as suggested by U.S. Pat. No. 3,946,751. None of the prior art known to me relating to cell casings and cathodic current collectors has suggested a boronized surface in contact with the cathodic reactant.
Utilization of boron in sodium/sulfur electrochemical cells or batteries has been suggested for specific applications. For example, U.S. Pat. No. 4,091,151 refers to boron oxide modified beta-type alumina for use as the cation-permeable barrier between the anodic reactant and cathodic reactant. U.S. Pat. No. 4,127,705 teaches the addition of selenium, boron or iodine to the cathodic reactant to obtain lowering of the viscosity of the sulfur thereby increasing rechargeability. U.S. Pat. No. 4,002,806 recognizes the problems of decreased cycle life and deteriorating charge/discharge capacity due to corrosion of metal cell parts and suggests use of protective liners and noncorroding, nonconductive material such as glass or ceramic while providing electric contact between the electrode and the external circuit by current lead rather than through the container body. U.S. Pat. No. 4,002,806 also suggests a boron oxide modified beta-type alumina ceramic material as a reaction zone separator. According to the U.S. Pat. No. 4,002,806, increase of the charge/discharge capacity of the sodium/sulfur battery or cell may be increased by addition to the cathodic reactant of metals from Groups I, II and III of the Peroidic Table of Elements, Transition Series Metals and antimony, lead, tin and bismuth and/or alloys, salts, oxides, phosphides, arsenides, antimonides, carbides and nitrides of the metals and/or mixtures thereof. To my knowledge the prior art has not suggested use of a boronized surface in contact with the cathodic reactant of a sodium/sulfur cell or battery and the utilization of an additional boron source in physical proximity to the boronized surface to obtain long-term stability and freedom from corrosion.